This invention relates generally to dynamoelectric machines, and in particular to an integrated starter generator having a rotor of decreased length to meet space constraints while maintaining efficient performance.
Dynamoelectric machines such as electric motors convert energy between electrical current and mechanical motion. As known to those skilled in the art, a machine typically includes a rotatable assembly, or rotor, mounted within a hollow core of a stationary assembly, or stator, which holds windings of insulated wire. The windings are energized with electrical current, causing the stator to magnetically interact with the rotor. The rotor and stator have cross-sectional shapes and sizes configured to minimize radial spacing between rotor and stator (known as the air gap), while avoiding any direct physical contact which could interfere with free rotation of the rotor.
Frequently, a machine must fit within a relatively small space, such as for installation inside an appliance, and yet maintain output or operational efficiency more closely commensurate with larger machines. Unfortunately, size constraints can reduce the generation of magnetic flux and torque. Typically, the stator core and rotor are each formed of a large number of flat, thin laminations of ferromagnetic material (e.g., steel) which are stacked to a desired length and secured together. The strength of the magnetic field and resulting output of the machine depend in part upon the size of the stator, and particularly its length. For a fixed level of electrical current in the windings, the torque output of a motor having a relatively longer stator is greater than the torque output of a motor having a relatively shorter stator.
Conventionally, the rotor and stator core have lengths which are equal. During manufacture, corresponding laminations for use in forming the rotor and stator core are die punched simultaneously in pairs from flat, thin-gauge sheets of material. Typically, all of these pairs of laminations are used to stack the stator core and rotor to equal lengths because they each contribute to an improvement in the air gap permeance (i.e., the ability to conduct magnetic flux between the stator core and rotor). As known to those skilled in the art, permeance increases proportionally with the length of overlap between the stator core and rotor (i.e., the length of the air gap). A motor having a rotor and a stator core of differing lengths will generally conduct flux primarily along the overlap. Another reason previous machines have equal length rotor and stator core is that it is generally considered a waste of ferromagnetic material to discard laminations in forming the shorter component, as the laminations are available in corresponding pairs.
The need to package some machines and associated electronic components within an extremely compact space demands that the machine have a length which is small. For example, an integrated starter generator for an automotive vehicle must be packaged so that it occupies only a relatively small volume in a vehicle engine compartment (e.g., maximum length and diameter dimensions of about six inches). These size constraints are superimposed on the need for highly efficient operation, and can produce conflicting design guidelines. Although a reduction in stator core length would better meet space constraints, that would reduce torque output or, alternatively, require an increase in electrical current to maintain equivalent magnetic flux and torque. The latter degrades efficiency because additional electrical current brings an increase in iron and copper losses and related heat generation. Further, the ferromagnetic materials cannot support infinite magnetic flux densities. They tend to “saturate” at a certain level (dictated by the dimensions and geometry of the part) such that further increases in magnetic field force do not result in proportional increases in magnetic field flux. When the stator becomes magnetically saturated, an increase in electrical current fails to produce a substantial increase in torque output.
Further aggravating the design requirements is the need to provide sufficient space for heat transfer in cooling the machine. The machine generates heat during operation which may degrade efficiency or damage component parts, such as the windings or bearings. One cooling approach which has been adopted is to install a cooling jacket around the outer circumference of the stator core. The jacket receives a coolant which is circulated through passages in or around the jacket to remove heat generated in stator windings and core during operation. Cooling is particularly challenging for a relatively small or compact machine, which has less area for conduction of heat from the stator core to the cooling jacket. A reduction in stator core length degrades heat transfer capacity and can result in inadequate heat dissipation.